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I am studying heat and thermodynamics by zemansky. In the chapter entropy, there is a statement under the section 'Entropy and Irreversibility'-

In the case of any process involving the isothermal transformation of work W done by a system into internal energy of a reservoir there is no entropy change of the system because the thermodynamic coordinates of the system do not change.

I don't understand how the thermodynamic coordinates do not change. Only the temperature is constant isn't it?

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  • $\begingroup$ Do they give any examples of the kind of process they are alluding to here? $\endgroup$
    – Chet Miller
    Commented Dec 29, 2020 at 19:04
  • $\begingroup$ They have given examples such as- 1)Friction from two solids in contact with a reservoir 2)Irregular stirring of a viscous liquid in contact with a reservoir 3)Inelastic deformation of a solid in contact with a reservoir etc $\endgroup$
    – 1500kook12
    Commented Dec 30, 2020 at 8:54
  • $\begingroup$ So these are not cases in which work is done to change the volume of the system? $\endgroup$
    – Chet Miller
    Commented Dec 30, 2020 at 12:24
  • $\begingroup$ Oh! they mentioned the system remains unchanged. was that the reason why thermodynamic coordinates don't change? I didn't realize that. I focused only on the process being isothermal. $\endgroup$
    – 1500kook12
    Commented Dec 31, 2020 at 9:16

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The section 8.7 you are referring to starts with the sentence "Examples are those processes involving the isothermal dissipation of work through a system (which remains unchanged) into internal energy of a reservoir, such as, etc.,". Therefore it is assumed that $\Delta U=\Delta Q + \Delta W=0$ and then $\Delta Q = -\Delta W$, and then all internally and irreversibly generated entropy is being "pushed out", so to speak, from the system into the reservoir whose entropy will be increased by $\Delta S = -\frac{\Delta Q}{T}=\frac{\Delta W}{T}$

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It was a fun challenge to try to decipher this without having the book, but I believe I cracked the code.

I am fairly certain that he is talking about processes in which the initial state of the system is the same (thermodynamically) as the final state, not just the temperatures.

So this would be a process where energy somehow "flows" through the system, but at the end ends up in the reservoir, with the system ending up in the exact state it started from.

Here's an example: if I wave my arms in my room (which is let's say in thermal equilibrium with the outside), I will do some work against the air molecules, which will temporarily heat the room up a little bit. But, assuming the temperature outside stays constant, all that energy I added to the room will eventually dissipate into the outside, leaving the room in the exact state it was before.

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